Duplex & Super Duplex Stainless Steel: A Comparison

Stainless steel types are classified into several groups, mainly attributing to distinct microstructural phases. Among them, duplex stainless steel stands out due to its unique microstructure resembling two other classes - ferritic and austenitic stainless steels. Duplex stainless steels boast a mixed microstructure with equal proportions of austenite and ferrite, leading to very high pitting resistance equivalent number values (PREN). This group can be further classified, revealing the super duplex stainless steel.

Super Duplex vs. Duplex Stainless Steel

Super duplex variety of stainless steels distinctly offers supreme corrosion resistance with PREN values ranging between 38 – 45. Their chemical composition is richer than any regular stainless steel, e.g., grade 304 or 316. This superior alloying imparts robust corrosion resistance and high mechanical strength. Nonetheless, this high alloying is also responsible for making super duplex steel processing more challenging.

Intermetallic phases are formed more comfortably due to high contents of elements like chromium, nickel, molybdenum, nitrogen, and tungsten. While these phases reduce the impact resistance of super duplex stainless steel, this shouldn't be of concern if you're working with experienced manufacturers that promise high-quality products.

Cost Disparity between Duplex and Super Duplex

Despite duplex and super duplex having better corrosion resistance than austenitic stainless steel alloys, super duplex particularly displays higher resistance to pitting. This quality is achieved by raising chromium content to approximately 27%, triggering changes in the alloy’s dual microstructure. Consequently, super duplex's chemistry will contain higher amounts of nickel alongside other elements, affecting cost due to nickel's volatility and molybdenum's rarity.

Welding Duplex and Super Duplex Stainless Steels

Welding these becomes a task requiring experienced skills, as they’re affected differently than austenitic stainless steels. Heat affected weld zones (HAZ) in these alloys demonstrate loss in toughness, post-weld cracking, and corrosion resistance reduction. Thus, skillful welders using proper SAW, TIG, and MIG techniques coupled with correct post-welding processing are essential.

Corrosion Resistance of Duplex and Super Duplex

The addition of chromium, nickel, molybdenum, nitrogen, and tungsten improves their corrosion resistance. They are effective at high temperatures without oxidation corrosion and show excellent resistance to various alkaline, acidic, and neutral solutions.

Machining Duplex & Super Duplex Stainless Steels

These metallic materials tend to work hard, thus specific parameters are necessary when machining them. Considering the alloys' high strength and low thermal conductivity, stronger and rigid machines, high-performance inserts, and a high quantity of coolant are advisable. Proper speeds and feeds also enhance efficient machining of duplex and super duplex stainless steel alloys.

Through careful assessment, it becomes clear that both duplex and super duplex stainless steels have their unique strengths and specific uses. Judicious selection can lead to improved material lifespan, better performance, and overall cost savings.